<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title></title>
    <script>
    document.onclick=function (){
      let a=()=>{
        alert(this);
      };

      let oBtn=document.getElementById('btn1');
      oBtn.onclick=a.bind(12);
    };
    </script>
  </head>
  <body>
    <input type="button" value="aaa" id="btn1">
  </body>
</html>
